Which of the following oxoacid of sulphur has -O-O- linkage?
(1) H _{2} S _{2} O _{8}, peroxodisulphuric acid
(2) H _{2} S _{2} O _{7}, pyrosulphuric acid
(3) H _{2} SO _{3}, sulphurous acid
(4) H _{2} SO _{4}, sulphuric acid